Spectra Of Quantum Dots Research Articles

Spin-valley-resolved energy spectra of quantum dots in the graphene/transition metal dichalcogenides system

We study the energy spectra of Dirac fermions confined in the quantum dot formed in the graphene/transition metal dichalcogenides (TMD) system under a perpendicular magnetic field. The obtained spin-valley polarized spectra can be classified into three types according to the values of the energy gap, which strongly depends on the pseudospin potential and the valley-dependent spin-orbit coupling peculiar to the electronic structure of the graphene/TMD system. For the first case, the electrons for both valleys and both spins possess linear dispersion. The energy spectra of the quantum dot exhibit sudden jumps of levels due to the Berry phase jumps. For the second case, the electrons for the $K({K}^{\ensuremath{'}})$ valley with up (down) spin have linear dispersion while the electrons for the $K({K}^{\ensuremath{'}})$ valley with down (up) spin have parabolic dispersion. The sudden jumps and the degeneracy-splitting-recombination behaviors of levels coexist in the energy spectra. The latter behavior is a result of the continuous change of the Berry phase. For the third case, the electrons for both valleys and both spins have parabolic dispersion. The degeneracy-splitting-recombination behaviors make the energy spectra both spin and valley distinguished. In addition, the evolutions of the levels from the sudden jumps to the continuous changes by adjusting the valley-dependent spin-orbit coupling and the pseudospin potential are also investigated. These results provide the possibility to control the specific spin-valley freedom of electrons in the graphene/TMD system using the quantum dot setup.

Characterizing Raman modes and gas sensing features of functionalized tetragonal graphyne quantum dots: A first principles study

This theoretical work has availed the first-principles method to critically explore the characterizing Raman spectra of tetragonal graphyne quantum dots. The Raman spectra of these T-graphyne systems possess the fingerprint of both sp and sp2 bonded atoms. We have explored that sp atoms predominate in determining intense Raman peaks over 2000 cm−1. Group theoretical analysis also provides a different insight into the occurrence of Raman active modes in these systems. Furthermore, the energy preference guides us to substitutionally dope boron (B) and nitrogen (N) atoms into the most stable α-T graphyne structure. The doping process enhances the system's stability and reduces the energy gap to a value of 1.29 eV. The concentration and position of the BN pair in T graphynes can be identified from the intense BN stretching mode at ∼ 1890 cm−1. Moreover, angle-dependent polarized Raman analysis depicts the effect of parallel and perpendicular polarization on the vibrational modes of T-graphyne for different incident laser wavelengths of 532 nm, 633 nm, and 785 nm. The anisotropy in the period of polarization-dependent intensities can be used to identify the crystal orientation direction. Firm absorption peaks in the UV–Vis spectra are also discussed from the allowed electronic transitions. Furthermore, the BN doped α-T graphyne structure possesses a large dipole moment of 5.2 Debye and can adsorb carbon-monoxide (CO) molecules with fascinating recovery time. The I–V response of the adsorbed T-graphyne system exhibit reasonably good sensing behaviour within a bias range of 0.28–0.29 V. Our results on vibrational signatures, and gas sensing applications will serve as an essential benchmark for graphyne-related research.

Recent advances in eco-friendly quantum dots-based solar energy conversion applications

<p indent="0mm">Semiconductor quantum dots (QDs) are nanocrystals with three-dimensional confined excitons, showing size/shape/composition-dependent optical properties with broadband absorption from ultraviolet to near infrared, high quantum yield, large Stokes shift and good photo/chemical stability, which have been widely applied in solar energy conversion applications such as solar cells (SCs), luminescent solar concentrators (LSCs) and photoelectrochemical (PEC) cells. However, most of the high-performance QDs-solar energy conversion devices are still based on QDs containing highly toxic heavy metals (such as Pb, Cd, Hg-based chalcogenides), which can inevitably induce the human health and environmental pollution issues, thus hindering their future commercialization. Developing high-efficiency solar energy conversion devices based on environmentally friendly QDs is a promising research direction to promote the practical application. Towards this effort, a new generation of environment-friendly semiconductor QDs (such as carbon QDs, silicon QDs, III-V compound QDs, I-III-VI compound QDs and lead-free perovskite QDs) has recently attracted extensive research interests. Here, notable advances and developments of solar cells, luminescent solar concentrators and photoelectrochemical cells based on these environmentally friendly QDs are summarized. Various strategies including band gap engineering, core/shell structure construction, doping, defect states tuning and alloying of these eco-friendly QDs as well as relevant QDs-based device performance are discussed in detail. Specifically, growing core/shell structure can effectively passivate the surface defect states, inhibit the non-radiative recombination and improve the photoluminescence quantum yield (PLQY) as well as photo-/chemical-stability of QDs. The optical absorption and PL spectra of QDs can be precisely tuned by altering the concentration of dopants and chemical compositions of QDs to match the solar spectrum for high-efficiency utilization of solar energy. Alloying strategy also enables the realization of optimized intrinsic and surface defects, tailored band structure and improved photo-/chemical-stability of QDs, thus enhancing the performance of corresponding QDs-solar conversion devices. Furthermore, the current existing challenges and drawbacks are analyzed, providing guidelines for future developments of eco-friendly QDs-based solar energy conversion devices. In summary, the rational design and synthesis of eco-friendly QDs with broad light absorption, highly efficient charge separation/transfer and outstanding photo-/chemical-stability are beneficial to improve the performance of “green” QDs-solar energy conversion devices. For QD’s synthesis, low-temperature, less toxic and large-scale synthetic technology with reduced energy consumption and eliminated toxic organic solvents/surfactants should be developed to achieve future cost-effective and high quality QDs. For QDs-SCs, the best reported power conversion efficiency of state-of-the-art QDs-SCs is still much lower than that of the commercialized silicon SCs. It is of great significance to explore more SCs device performance and stability optimization strategies. For QDs-LSCs, the balance between reabsorption loss and PLQY should be further studied and the standard measurement methodology needs to be refined. For QDs-PEC devices, the long-term durability of QDs under device operation needs to be largely improved for real-life application. Besides, the electrolyte used in most of the current QDs-PEC systems contains sacrificial agents (e.g., Na₂S/Na₂SO₃) with highly corrosive feature, which may induce environmental issues and should be properly replaced by neutral solution for future commercial perspectives.

Multi-layer programmable spectral anti-counterfeiting based on quantum dots inkjet printing.

High security and convenient operation have been the aim of the anticounterfeiting field. An anticounterfeit system in which multiple information are written on an overlapping area with quantum dot (QD) inks of different fluorescence wavelengths that combines spectroscopy technology for information identification is proposed in this study. Because the emission spectra of QDs can be tuned by simply changing the size, ultraviolet (UV) glue mixed with different sizes of CdSe/ZnS QDs is used as the printing ink with different fluorescence wavelengths. Software-design labels with different information are printed on the overlapping area using inks with different luminescent QDs. The printed information can only be identified by the bandpass filter with the corresponding wavelength under UV light. Under natural light, the information cannot be identified by the naked eye. In the proposed anticounterfeiting system, the excitation light and filter are both indispensables. Our method makes the fabrication of anticounterfeit labels flexible in design, fast in production, and high in information concealment. Meanwhile, the proposed system is quick and convenient, which has huge application potential in the field of display and anticounterfeiting.

Full-Color Quantum Dot Light-Emitting Diodes Based on Microcavities

Full-color display is a primary challenge for the commercialization of quantum dots (QDs). In this study, we utilize the spectral narrowing phenomenon of microcavities to fabricate the red, green and blue quantum dot light-emitting diodes (QLEDs) with a single QD layer. This work theoretically analyses the role of microcavities in adjusting the emitting color of QLEDs. By enhanced microcavity and properly chosen spacer thickness, the spectral selectivity shifts, realizing the full-color-tunability of QLEDs. The tunable experimental spectra of microcavity QLEDs are observed, in excellent agreement with our theoretical design. Benefiting from the spectral narrowing of microcavity and the narrow spectra of QDs, a high color purity with full width at half maximum (FWHM) of 18 to 25 nm is realized, leading to a color gamut ratio of 104.8&#x0025; compared to National Television System Committee (NTSC) standard. The light extraction is also enhanced by constructive interference and the Purcell effect in the microcavity. Moreover, in the fabrication of red, green, and blue pixels, patterning the transparent cathode has better feasibility and lower damage relative to patterning the light-emitting layer.

Temperature-Dependent Optical Properties of Perovskite Quantum Dots with Mixed-A-Cations.

In this work, metal halide perovskite quantum dots (QDs) with Formamidinium (FA) and Cs mixed cations were fabricated using a solution-processed method at room temperature. By controlling Cs doping ratios in a precursor, the optical properties of mixed-cation perovskite QDs were systematically studied. With the increase in Cs ion doping, the photoluminescence (PL) spectra of perovskite QDs were blueshifted, which was mainly due to the smaller radius of Cs ions than those of FA. Temperature-dependent PL spectra were conducted on mixed-cation perovskite QDs. As the temperature gradually increased from 4 K to 300 K, PL peaks were blue shifted, and full-width at half maximum (FWHM) was widened, which was directly related to lattice thermal expansion and the carrier-photon coupling effect under temperature variation. At the same time, excess Cs ion doping had a prominent influence on optical properties at low temperatures, which was mainly due to the introduction of detrimental defects in perovskite crystals. Therefore, it is particularly important to control doping concentration in the preparation of high-quality perovskite QDs and efficient photoelectric devices.

Strong coupling between colloidal quantum dots and a microcavity with hybrid structure at room temperature

The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.

Core-shell type-II spherical quantum dot under externally applied electric field

This study presents a simple one-band model within the effective mass approximation to describe the electric field impact on the energy structure and interband electron quantum transitions in type-II CdSe/ZnTe and ZnTe/CdSe spherical quantum dots. The dependencies of the oscillation strength and the light absorption coefficient on the externally applied electric field are calculated by the diagonalization method for spherical quantum dots of different sizes. It is shown that in the absence of an electric field and due to the spherical symmetry of nanosystem, only quantum transitions with Δl=0 (1se−1sh, 1pe−1ph, 1de−1dh, 1se−2sh,...) are allowed. The oscillator strength of quantum transitions between electron and hole excited states is greater than between the ground states. The electric field breaks the spherical symmetry, therefore, the oscillator strength of the 1se−1ph, 1se−1ph, 1se−1dh,... quantum transitions, which are forbidden for spherical symmetric systems, increase when electric field strength grows. Under electric field effects, the oscillator strength of allowed Δl=0 quantum transitions decreases. Finally, we observed that the absorption peaks in the quantum dot spectra under electric field influence are shifted to the low-energy region.

Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.

In-plane electric and exchange fields engineered corner state bismuthene quantum dots

The energy spectrum of bismuthene quantum dots in the presence of in-plane electric and exchange fields are investigated by the tight-binding method. In the case of an in-plane exchange field, bismuthine is a second-order topological insulator and supports corner states in triangular and hexagonal quantum dots with zigzag edges. The corner states in hexagonal bismuthene quantum dots are more stable than the corner states in triangular quantum dots. In hexagonal quantum dots, the corner states are degenerate at zero energy as the exchange field h > 0.18 eV. However, the degeneracy of corner states can be destroyed by the electrical fields, and the energy levels of corner states oscillate with respect to the electric field direction. In addition, the on-site disorders will not destroy the corner states but lead to the separation of these states around zero energy.

Investigation of the features of electronic spectrum of quantum dots in narrow-gap semiconductors

Samples with quantum dots (QDs) of narrow-gap semiconductors of the A3B5 group (indium antimonide) and the A2B6 group (mercury selenide) have been studied. The absorption spectra of the investigated QDs are analyzed and the correspondence of the maxima in the spectral characteristics to the model representations of the calculated electronic energy spectrum for this materials is assessed. It is concluded that used model representations requires refinement, primarily due to the fact that studied objects are nanocrystals with complex geometry. Keywords: quantum dots, indium antimonide, mercury selenide, electronic energy spectrum.

On the optical measurement of microparticle charge using quantum dots

We investigated the possibility of using a layer of quantum dots (QDs) deposited on the microparticle surface for the measurement of the charge the microparticle acquires when immersed into a plasma. To that end, we performed the calculations of the Stark shift of the photoluminescence spectrum of QDs caused by the fluctuating local electric field. In our calculations, we assumed the plasma-delivered surplus electrons to be distributed on the surface of a microparticle. According to our calculations, the Stark shift will acquire measurable values when the lifetime of the quasi-stationary configuration of the surplus electrons will be determined by their diffusion along the surface. Experiments with flat QD-covered floating plasma-facing surfaces suggest that measurable Stark shift of the photoluminescence spectrum can be achieved. Based on our model, modern microscopic plasma-surface interaction theories and analysis of the experiments, we suggest the possible design of the charge microsensor, which will allow to measure the charge accumulated on its surface by means of visible-light optics.

Majorana fermions induced fast- and slow-light in a hybrid semiconducting nanowire/superconductor device

We investigate theoretically Rabi-like splitting and Fano resonance in absorption spectra of quantum dots (QDs) based on a hybrid QD-semiconducting nanowire/superconductor (SNW/SC) device mediated by Majorana fermions (MFs). Under the condition of pump on-resonance and off-resonance, the absorption spectrum experiences the conversion from Fano resonance to Rabi-like splitting in different parametric regimes. In addition, the Fano resonances are accompanied by the rapid normal phase dispersion, which will indicate the coherent optical propagation. The results indicate that the group velocity index is tunable with controlling the interaction between the QD and MFs, which can reach the conversion between the fast- and slow-light. Fano resonance will be another method to detect MFs and our research may indicate prospective applications in quantum information processing based on the hybrid QD-SNW/SC devices.

Highly selective and sensitive detection of glutathione over cysteine and homocysteine with a turn-on fluorescent biosensor based on cysteamine-stabilized CdTe quantum dots

In this work, cysteamine (CA) stabilized CdTe quantum dots (QDs) (CA-CdTe QDs) and sodium citrate stabilized gold nanoparticles (AuNPs) were prepared. Because of the strong electrostatic interaction and spectral overlap of emission spectrum of CA-CdTe QDs and absorption spectrum of AuNPs, a highly effective fluorescence resonance energy transfer (FRET) system was formed and the fluorescence of CA-CdTe QDs was strongly quenched. The synthesized CA-CdTe and AuNPs were self-assembled to large clusters due to the electrostatic attraction and the fluorescence of CA-CdTe was sharply quenched as a result of FRET. Under the optimum pH of 5.5, the positive GSH could assemble with negative AuNPs through electrostatic interaction and destroy the FRET system of CA-CdTe and AuNPs, due to the red shift of absorption wavelength of AuNPs caused by aggregation. The fluorescence of CA-CdTe recovered, and the recovered fluorescence efficiency shows a linear function against the GSH concentrations from 6.7 nM to 0.40 μM, with a detecting limit of 3.3 nM. The quenched emission of CA-CdTe could be recovered attributed to the aggregation of AuNPs by GSH. Under optimal conditions, the sensing system was successfully applied in the detection of GSH in real human blood plasma samples with a recovery of 99.5–102.3%, showing a promising future for the highly sensitive and selective GSH detection in the human blood plasma samples.

Ultrasensitive prostate specific antigen monitoring based on electrochemiluminescent immunesystem with synergistic signal amplification effect of resonance energy transfer coupling with K2S2O8-H2O2 dual coreactants

• The synergistic effect of coupling dual coreactants with the surface plasmon resonance of AgNPs enhancement was used as an effective signal amplification strategy. • OH • arising in the process was able to induce the decomposition of S 2 O 8 2− into SO 4 • − resulting in a vigorous ECL enhancement. • The amplified ECL immunosensor was fabricated for highly-sensitive detection of PSA. • The sensing system provides an alternative charming method for other biomolecules assays. In this work, an effective signal amplification strategy was used for the fabrication of the ultrasensitive sandwich-typed electrochemiluminescence (ECL) immunosensing platform. The combining of K 2 S 2 O 8 -H 2 O 2 dual coreactants with the localized surface plasmon resonance of silver nanoparticles (AgNPs) resulted in the remarkable increase of ECL response. Noticeably, because of the perfect overlap between the plasmon spectra of AgNPs and the ECL emission spectra of graphitic carbon nitride quantum dots (g-CNQDs), a plasmon-enhanced ECL phenomenon was proved in g-CNQDs/AgNPs system result from high efficient energy transfer. Besides, the ECL signal of the ECL immunosensor was greatly improved in the presence of K 2 S 2 O 8 -H 2 O 2 dual coreactants. Consequently, this synergistic amplified ECL sensing platform was applied for the specific prostate specific antigen (PSA) analysis. Under the optimum conditions, the immunoassay strategy demonstrated excellent sensing performance with a wide linear concentration of PSA from 1.0 × 10 − 5 ng mL −1 to 500 ng mL −1 and a detection limit of 0.0046 pg mL −1 (S/N = 3). Considering the favorable analytical feedback in actual serum samples assay, the developed strategy indicated an enormous potential for biomarkers analysis.

Core@shell quantum dots as a fluorescent probe for the detection of cholesterol and heavy metal ions in aqueous media

In this paper, CuInS2@ZnS ([email protected]) core-shell quantum dots (QDs) were applied for the detection of heavy metals and bioanalytes. These Glutathione (GSH) capped [email protected] QDs with an average size of 3.8 nm were treated using different biologically important biomolecule analytes, such as ascorbic acid (AA), citric acid (CA), oxalic acid (OA), cholesterol (Ch), cysteine (Cy), dextrose (Dx) and urea (U). According to spectroscopic analysis, the addition of cholesterol showed photoluminescence (PL) spectra of QDs remains unchanged at the main peak position 600 nm but generates a new peak at 487 nm with cholesterol dependent enhancement. Furthermore, the addition of citric acid contributes to PL quenching in the core-shell QDs without any spectral shift. Likewise, the interaction of [email protected] QDs with different physiologically important heavy metal ions (HMI) was systematically studied as a luminescence probe. It was found out that the PL emission of [email protected] QDs was effectively quenched by copper (Cu2+) ion with significant spectral shift detected. However, Hg2+ ions also lead to PL quenching at nanomolar level but with no spectral shift observed. According to these results, [email protected] QDs are selectively sensitive to cholesterol, citric acid, Cu2+ and Hg2+ ions with satisfactory detection limit at nM level.

Solving Quantum-Dot Excitonic Riddles with Absolute Pump-Probe Spectroscopy.

Absolute absorption changes in molecular flash photolysis experiments are routinely translated into molar extinction coefficients and oscillator strengths of reactive intermediates. These direct quantum chemical investigation and allow precise concentration readings in later experiments. In this Perspective we show how a similar approach can deliver crucial information for interpreting transient absorption spectra in colloidal semiconductor quantum dots. The intrinsic complexity of such samples stemming from the inhomogeneity of particle size, shape, and surface chemistry poses unique challenges to mechanistic assignment of ultrafast pump-probe measurements. We will describe applications of this approach to elucidate the photophysics of quantum confined nanocrystals made of various semiconducting materials. These case studies demonstrate how, faced with conflicting interpretations, it has pointed in the right direction in assessing single and multiple exciton generation and relaxation, in searches for ultrafast carrier trapping and scavenging, and in tests of band edge level structure and state degeneracies.

Spin-valley coupling in single-electron bilayer graphene quantum dots

Understanding how the electron spin is coupled to orbital degrees of freedom, such as a valley degree of freedom in solid-state systems, is central to applications in spin-based electronics and quantum computation. Recent developments in the preparation of electrostatically-confined quantum dots in gapped bilayer graphene (BLG) enable to study the low-energy single-electron spectra in BLG quantum dots, which is crucial for potential spin and spin-valley qubit operations. Here, we present the observation of the spin-valley coupling in bilayer graphene quantum dots in the single-electron regime. By making use of highly-tunable double quantum dot devices we achieve an energy resolution allowing us to resolve the lifting of the fourfold spin and valley degeneracy by a Kane-Mele type spin-orbit coupling of ≈ 60 μeV. Furthermore, we find an upper limit of a potentially disorder-induced mixing of the K and K^{prime} states below 20 μeV.

OPA-driven hollow-core fiber as a tunable, broadband source for coherent multidimensional spectroscopy.

Despite the impressive abilities of coherent multi-dimensional spectroscopy (CMDS), its' implementation is limited due to the complexity of continuum generation and required phase stability between the pump pulse pair. In light of this, we have implemented a system producing sub-10 fs pulses with tunable central wavelength. Using a commercial OPA to drive a hollow-core fiber, the system is extremely simple. Output pulse energies lie in the 40-80 μJ range, more than sufficient for transmission through the pulse shaping optics and beam splitters necessary for CMDS. Power fluctuations are minimal, mode quality is excellent, and spectral phase is well behaved at the output. To demonstrate the strength of this source, we measure the two-dimensional spectrum of CdSe quantum dots over a range of population times and find clean signals and clear phonon vibrations. This combination of OPA and hollow-core fiber provides a substantial extension to the capabilities of CMDS.

Retraction Note: Fluorescence spectrum–based dual-parameter detection method for pH and DO in cancer cell metabolic fluid

Abnormal metabolism of cancer cells leads to changes in the concentration of various components in the cell metabolic fluid, which is obviously different from that in normal cells. The two most representative parameters used for the early diagnosis of cancer are acidity (pH) and dissolved oxygen (DO) in the external environment of cancer cells. This study aimed to propose a dual-parameter detection method to simultaneously determine the pH and DO of cancer cell metabolic fluid based on the fluorescence spectrum of coumarin-modified CdSe quantum dots. Quantum dot is an important low dimensional semiconductor material with nanometer level. The coumarin-modified CdSe quantum dots can be ultraviolet-excited with an excitation light of wavelength 365 nm. The realization of detecting pH and DO was demonstrated by characterizing the fluorescence intensity at 625 nm produced by CdSe quantum dots and at 485 nm induced by coumarin upon excitation. The results of the experiment with phosphate-buffered saline showed that the detection limit of pH was 0.05 and that of DO was 0.04 mg/L. Based on this, the pH and DO of the extracellular environment of cancer and normal cells were tested and compared to verify the accuracy of the measurement.The method proposed in this study could realize the dual-parameter detection of the cell metabolic fluid with high measurement accuracy, high speed, and less loss of raw liquid. The method could also be applied to other related detection fields.